In conventional small transformers having a high-insulation performance, the coils (windings) of a transformer body is coated with resin (polymer material). Such a transformer has outside dimensions having a certain precision by putting a transformer body in a transfer metal die and applying transfer molding to resin, or by accommodating the transformer body in a case, filling the case with resin in vacuum, and heating the resin to harden it. Transfer metal dies are used for manufacturing transformers that are unlikely to be affected by resin contraction, such as toroidal transformers, while cases are used for manufacturing transformers having a highly-sensitive magnetic material, such as permalloy transformers.